1. Field of the Invention
This invention relates to the construction of a bell jar in a cylindrical apparatus for the growth of epitaxial layers.
2. Description of the Prior Art
The cylindrical apparatus for the growth of epitaxial layers has such a construction as illustrated in FIG. 7. Specifically, it comprises a bell jar 21 made of quartz, a susceptor 22 disposed rotatably inside the bell jar 21, circularly depressed pockets 23 formed on the lateral wall surfaces of the susceptor 22, and a heater 24 surrounding the peripheral surface of the bell jar 21 and, by a procedure which consists of charging the pockets 23 each with a substrate (not shown) like a silicon single crystal substrate, causing the heater 24 to heat the susceptor 22 and the substrates through the bell jar 21, and keeping the susceptor 22 in rotation and meantime causing a raw material gas 25 for epitaxial growth to flow down the interior of the bell jar 21, effects growth of epitaxial layers one each on the substrates.
In the lateral wall portions R of the bell jar 21 that confront the substrates on the susceptor 22 in the apparatus for epitaxial growth described above, the bell jar 2 assumes the shape of an erect cylinder and the susceptor 22 assumes the shape of a prismoid (enclosed laterally with trapezoids) having a lower base surface larger than the upper base surface so as to retain the substrates fast in place. As illustrated in FIG. 9, therefore, the gap area between the peripheral surface 22a of the susceptor 22 and the internal wall surface 21a of the bell jar 21 (the area enclosed with the polygon forming the periphery of the susceptor and the circle forming the internal wall of the bell jar) increases upwardly and decreases downwardly in the lateral wall portions R of the bell jar. Incidentally, when the cylindrical apparatus for the growth of epitaxial layers is used to obtain growth of epitaxial layers one each on the substrates set in place therein, the distribution of film thickness of each substrate within the surface thereof is generally determined with the maximum and minimum values of film thickness found at a total of five points, i.e. the central point 1 plus the four points taken in the peripheral region of a given substrate 31 as illustrated in FIG. 8 [through the measurement with a Fourier transform infrared spectrophotometer (hereinafter referred to as "FTIR")]. The expression "four points in the peripheral region" as used herein refers to the four points 2, 3, 4, and 5 taken as regularly spaced circumferentially on a concentric curve C located at a distance of 5 mm from the contour of the substrate indicated by a broken line in FIG. 8. These points are so fixed that the point 4 will fall directly below the central point of an orientation flat part 31a and the straight line connecting the points 4 and 2 and the straight line connecting the points 3 and 5 will perpendicularly intersect each other.
Only naturally, the maximum values and minimum values of film thickness of a given epitaxial substrate at the five points mentioned above are desired to be as close to each other as possible. When the film thickness of an epitaxial layer on a substrate in the cylindrical apparatus for the growth of epitaxial layers is discussed, the distribution of film thickness in the direction of rotation of the susceptor raises a problem more often rather than the distribution in the direction of the major axis of the susceptor. When the substrate 31 is set in place with the orientation flat part 31a thereof held on the upper side as illustrated in FIG. 8, it is necessary to control the film thickness particularly at the points 5, 1, and 3.
For the sake of this control, the dishing ratio D which is defined by the following formula 1 is adopted at times if not always as a criterion of control. Naturally, this dishing ratio D is desired to be as close to 1 as possible.